1. Field of the Invention
The present invention relates to a valve actuator adapted to open and close a valve, and more particularly, to a vertical spring return valve actuator that makes use of one planetary gear unit.
2. Background of the Related Art
A spring return valve actuator is designed to automatically open fans when power is turned off by a fire, thereby preventing the death from suffocation. The valve actuator working in a state of emergency is largely classified into a pneumatic type valve actuator that works with the force of compressed air and a motor drive type valve actuator that works with electricity. Since the pneumatic type valve actuator should need air pressure at the installation site thereof, the motor drive type valve actuator that easily works with the application of the electricity thereto is used widely.
The pneumatic type valve actuator makes use of a spiral spring as a driving force in case of emergency, and to the contrary, the motor drive type valve actuator makes use of emergency batteries as well as the spiral spring. On sites, generally, the spiral spring is more preferred than the batteries. In case of the motor drive type valve actuator, the spiral spring is directly mounted on a valve, so that an electronic brake is released at the time of emergency like a blackout to permit the valve to be actuated by means of the spiral spring. So as to directly activate the valve, in this case, the spiral spring should have relatively high tension, which undesirably needs skilled workers for manufacturing process and raises the production costs.
A representative example of conventional valve actuators is disclosed in Korean Patent No. 10-0734394 that is issued to the same applicant as this invention. The valve actuator discloses that the power of a motor is transmitted from a first worm to a first worm wheel and then transmitted to a second worm fixed around a concentric shaft to the first worm wheel, thereby permitting a valve connected to a second worm wheel operated cooperatively with the second worm to be opened and closed. In more detail, as shown in FIGS. 9 and 10, a valve actuator 1 includes a body 11, a motor 15 disposed inside the body 11, a first worm gear 10, a first planetary gear unit 30, a second planetary gear unit 40, a second worm gear 20, a steering wheel worm wheel 72 and a spiral spring 55. A sun gear 31 of the first planetary gear unit 30 is mounted around a brake shaft 60 working a brake at the time of application of power. At the time of normal operation, accordingly, since the sun gear 31 of the first planetary gear unit 30 mounted around the brake shaft 60 is fixed, the power applied from the motor 15 is transmitted sequentially to the first worm gear 10, a ring gear 35 and planetary gears 33 of the first planetary gear unit 30, a ring gear 45, planetary gears 43 and a sun gear 41 of the second planetary gear unit 40, and a second worm 21 and a second worm wheel 23 of the second worm gear 20, thereby permitting a valve (not shown) disposed on the second worm wheel 23 to be opened and closed. At the time of emergency operation wherein power is turned off, on the other hand, the brake shaft 60 is released to permit the force applied from the spiral spring 55 to actuate the second worm 21 and the sun gear 41, so that the closed valve becomes opened automatically. Further, at the time of emergency operation wherein power is turned off and the motor 15 stops, even in the state where the valve is opened by the force of the spiral spring 55, the steering wheel worm wheel 72, the second planetary gear 43 and the second worm 21 are activated by means of a stopper wheel 71 and a steering wheel 77 to permit the valve to be opened manually.
However, the conventional valve actuator needs a spring restoring force having a substantially large capacity, e.g., 500 Nm to 600 Nm and has an arrangement structure wherein two planetary gear units 30 and 40 are disposed in a lengthwise direction, thereby causing the whole length to be extended, so that the size of the actuator becomes bulky and the number of parts becomes increased.